Worldwide food shortages over the past several years have encouraged the research and development of methods and apparatus for producing high quality, low cost microbial protein, i.e., single cell protein (SCP), to alleviate the food shortages.
Efforts to relieve the worldwide shortages of protein have included various biosynthesis processes. Biologically produced single cell protein (SCP) has been obtained by the growth of a variety of microorganisms including bacteria, yeast and fungi on a variety of carbon-containing substrates. Petroleum hydrocarbons have been employed as carbon and energy sources, but have faced practical difficulties due to their lack of water solubility, and their high consumption of oxygen required for microbial conversion. Other feedstocks used include oxygenated hydrocarbon derivatives due to the inherent water solubility of such feedstocks. Oxygenated hydrocarbon derivatives are commonly employed because of their ease of handling since microbial conversion processes are essentially conducted under aqueous conditions.
Aerobic microbial conversions are highly exothermic oxidation reactions which demand large quantities of molecular oxygen, and which produce large quantities of heat. The heat must be removed continuously and consistently, at the risk of overheating the system. Overheating of the system can cause the death of the microorganisms, or at least cause severe limitations on growth of the microorganisms as temperatures rise, and hence cause severe reductions in fermentor efficiencies. In addition, sufficient supplies of molecular oxygen are necessary to maintain high fermentation efficiencies.
To maintain high fermentation efficiencies in commercial fermentations, oxygen is supplied to the culture media as a molecular oxygen-containing gas free of any stray microorganisms under conditions to provide maximum contact of the oxygen with the culture media. This is done to dissolve as much oxygen in the aqueous media as possible.
High oxygen transfer rates have been achieved heretofore by conducting a fermentation process as a foam-type process. The use of foam assists in achieving a high surface area for contact between the liquid phase and the gas phase. Foam also allows one to obtain a high rate of oxygen transfer from the gas phase into the aqueous phase, and at the same time assists in obtaining a good rate of removal of carbon dioxide, a natural consequence of aerobic fermentation processes. Carbon dioxide which is transferred from the aqueous medium to the gas phase is then exhausted for such use as may be suitable.
There is a continuing need for improved apparatus suitable for conducting aerobic fermentation processes with high oxygen transfer rates, i.e., apparatus capable of providing effective contact between the aqueous medium and the oxygen-containing gas phase. At the same time, an apparatus is desired which is basically straightforward in construction, economical to manufacture and maintain, and yet well adapted for its intended use.
Research has been conducted for processes which provide for growing SCP at high productivities, e.g., to high cell densities as disclosed in U.S. Pat. No. 4,414,329 by Wegner (assigned to Phillips Petroleum Company). High productivity may be defined as fermentor productivity of at least 8 grams per liter per hour (g/L/hr) based on the volume of ungassed broth. High cell density may be defined as the growth of microbial cells (i.e., typically bacteria and yeast) in unusually high concentrations (i.e., .about..gtoreq.50 g/L for bacteria and .about..gtoreq.80 g/L for yeast). The obvious advantage, of course, of growing SCP to high productivities is that there is provided a greater quantity of food source from a given fermentor volume for a given period of time.
As a consequence of the development of high productivity fermentation processes, there is an even greater need for improved aerobic fermentation apparatus for the continuous production of microbial cells at high productivities. A well designed apparatus will, of course, optimize several features or properties of the fermentation apparatus. For example, extremely high heat removal and oxygen transfer capabilities are necessary. Lower power inputs to operate the fermentation apparatus while still achieving the above desired ends is also highly desirable. Finally, a fermentation vessel which does not require an anti-foaming agent for effective control of a high cell density fermentation process would provide a tremendous advantage. This is because the presence of anti-foaming agent causes not only reduced oxygen transfer but also results in an increased density of fluid mass, which in turn requires increased power input. Moreover, anti-foaming agent ends up in the final product, which inclusion may be undesireable.
Thus, a fermentation apparatus which can be used in aqueous, aerobic fermentation process, particularly high productivity processes, and that maximizes and/or minimizes the above-mentioned features, as appropriate, is highly desirable.